An angle detecting device is attached to a rotation shaft in order to detect the number of revolution and a rotation angle of the rotation shaft.
As for an apparatus for performing a control operation by using the angle detecting device, there is known a servo apparatus for use in a radio-controlled model (RC model), an industrial machine, a robot or the like.
The servo apparatus performs feedback control based on a control signal received from a controller and a rotation angle of a rotation shaft. The rotation shaft serves as an output shaft and the rotation angle of the rotation shaft is obtained by the angle detecting device. Further, the servo apparatus performs a control operation such that the rotation angle of the output shaft is equal to a rotation angle received from the control signal.
Particularly, the servo apparatus for use in a remotely controlled apparatus such as a RC model or the like is connected to a receiver and operates based on a control signal transmitted from the controller to the receiver.
In the angle detecting device of the servo apparatus for use in the RC model, a potentiometer has been conventionally used as a detecting element. The potentiometer uses a variable resistor and detects a rotation angle of the output shaft based on an output voltage that varies depending on the rotation angle of the output shaft.
The potentiometer is widely used as a detecting element due to its cost effectiveness. The potentiometer uses a variable resistor and thus includes a resistor and a brush. Since the resistor and brush operate in contact with each other, the resistor or a sliding portion is worn as they are used for a long time. Especially, when the resistor is worn, a resistance is changed, which may result in deviation of the reference position of the output shaft or misdetection of the rotation angle of the output shaft.
Therefore, an angle detecting device using a non-contact type magnetic sensor as a detecting element and a servo apparatus using the same have been developed and disclosed in Japanese Patent Application Publication No. 2011-030376.
Hereinafter, a configuration of the servo apparatus using the magnetic sensor which is disclosed in Japanese Patent Application Publication No. 2011-030376 will be described with reference to FIG. 7. A servo apparatus 20 includes a motor 21, speed reducers 22 for reducing a rotation speed of the motor 21 and transferring the reduced rotation, an output shaft 23 for transferring the torque of the final gear of the speed reducers 22, a magnet 24 connected to an end portion of the output shaft 23, a magnetic sensor 25 provided inside a case, and a control unit (not shown) provided inside the case.
The servo apparatus disclosed in Japanese Patent Application Publication No. 2011-030376 uses a Hall element as a magnetic sensor. Since the Hall element and the magnet are not in contact with each other, components thereof are not worn even if they are used for a long time, which removes the problem of the potentiometer becoming unable to detect the rotation angle.
However, in the case of using a magnetic sensor, a detectable rotation angle is generally narrower compared to that in the case of using a potentiometer. An example employing a Hall element as a magnetic sensor will be described with reference to FIGS. 4A and 4B.
FIG. 4A illustrates a circular plate-shaped magnet 7′ and a magnetic sensor 8 when seen in the axial direction of an output shaft 5′. Points 7′a, 7′b, 7′c, and 7′d are provided on a circumference of the magnet 7′ such that they are spaced apart from each other at an interval of 90°. A point on the circumference of the magnet 7′ which is closest to the magnetic sensor 8 is set to a position of the magnet. The relation between the position of the magnet and the output voltage is shown as a comparative example 1 in FIG. 5C. Referring to FIG. 5C, the output voltage with respect to the position of the magnet is linearly changed within a range of ± about 45° from the point 7′a. In such a range, the rotation angle can be detected.
On the other hand, in a servo apparatus for use in a RC model, it is desired to detect a rotation angle within a range of ± about 60° from the reference position. In a servo apparatus for use in an industrial machine, it is desired to detect a rotation angle within a range of ± about 75° from the reference position.